1. Field of Invention
The present invention relates generally to power amplifiers and, more particularly, to power amplifiers for multi-band, multi-standard transceivers.
2. Discussion of Related Art
Typical radio frequency (RF) power amplifier circuits use impedance-varying circuitry, such as an impedance transformer, impedance matching circuit or tank circuit, at their outputs in order to enhance performance. An example of this configuration is illustrated in FIG. 1. The RF power amplifier circuit 100 includes an input circuit 102 coupled between an input terminal 104 where the RF signal to be amplified is applied, and the base (or gate) of the power transistor 106. A bias current, IBIAS, is provided to the transistor 106 from a supply voltage terminal 108 via an inductor 110, and an output voltage, VOUT, is generated across the transistor. The transistor 106 drives a load 114 via the output circuit 112, with a load current, ILOAD drawn by the load and a corresponding load voltage, VLOAD, generated across the load. The power provided to the load, PLOAD, is given by the following equation:PLOAD=VLOAD*ILOAD*cos(θ)  (1)Wherein θ is the phase angle between the load voltage, VLOAD, and load current, ILOAD, used to calculate the power, PLOAD.
The output circuit 112 is generally narrow-band in order to maximize performance of the power amplifier circuit 100 at a given frequency. However, in many modern applications, such as mobile telephone handsets or wireless network cards for computers, wide-band or multi-band operation may be required or desirable and therefore, the power amplifier should have good performance over a wide frequency bandwidth. Conventional attempts to increase the bandwidth of the output circuit (and thus the power amplifier circuit) using feedback control, multi-stage reactive matching and other techniques, decrease most power metrics of the power amplifier circuit, including the output power, PLOAD, delivered to the load, the efficiency, η (given by equation (2) below), the Mask (being the standards body-defined limits of spectral emissions for a transmitter, usually given in dBc/Hz or dBm/Hz), and the error vector magnitude (i.e., the difference between the desired symbol location in the constellation and the actual symbol location).